The invention relates to an adaptive CAP receiver and relates, in particular, to a method for driving individual control loops of an adaptive CAP receiver, in order to ensure stable start-up and stable operation of the receiver.
An ATandT and Bellcore article entitled: Design of Digital Carrierless AM/PM Transceivers, TEI.1.4/92-149, Aug. 19, 1992, provides an introduction to the layout of digital, carrierless CAP transceivers and receivers. Therefore, basic structures will only be described briefly herein.
When dealing with large amounts of data, a frequency-division multiplex method has been found to be better than a baseband duplex channel method with echo compensation for duplex data transmission through subscriber access lines. That is the case despite the greater bandwidth which is required, since crosstalk which is dominant in that speed range can be suppressed by selective filtering. Thus, for rapid data transmission in the telephone subscriber access area, single-carrier methods such as QAM (Quadrature Amplitude Modulation) and CAP (Carrierless Amplitude/Phase modulation) are currently being discussed under the keyword VDSL (Very high bit rate Digital Subscriber Line). That is because they allow the various frequency bands to be achieved for outward and return directions in a simple manner by a suitable selection of carrier and mid-frequencies. The data rates in that case vary over a range from about 2 Mbit/s to 50 Mbit/s. In that case, both symmetrical and asymmetrical operation are intended to be possible. Symmetrical operation, that is to say equal data rates in both directions, is generally required for commercial applications, while asymmetrical operation, that is to say different data rates in the two directions, is generally sufficient in the private area (high data rate to the subscriber, low data rate to the service provider).
Until now, the discrete multitone method has been standardized in the ADSL Standard (Asymmetric Digital Subscriber Line). However, it can be assumed that single-carrier methods using CAP technology will also be standardized in that application area.
In a CAP system, the binary data are assembled into groups of bits at the transmission end, and are supplied to a coder. That allocates a point in a two-dimensional signal space to each of the 2L combinations. That point is defined by Cartesian co-ordinates (x, y). That signal point may also be regarded as a point in complex number planes.
A CAP modulator will be initially considered. The signal points to be transmitted, which are defined by two Cartesian co-ordinates ak and bk at a time kT, are sampled at a clock rate (symbol rate) fT=1/T and are passed to separate sub-channels, each of which contains a transmission filter. The signals in the two sub-channels are subsequently added, and the transmission signal is passed to the transmission channel.
The received input signal is subjected to A/D conversion at a predetermined sampling rate in a corresponding CAP receiver, which forms a matching element to the CAP transceiver. Digital level regulation is carried out after the digital A/D conversion, in order to set the level of the received/wanted signal to a constant value. Therefore, as far as possible, it is independent of the transmission line and of the crosstalk of the transmission signal of the system, as well as all other out-of-band interference. The samples are passed to the pair of reception filters after digital level regulation. In that case, the coefficients of the filter pair must be adjusted adaptively, starting with a fixed coefficient set, which is configured for a specific line length. After reception filtering, the signal values are sampled at the symbol rate fT, and are passed to a decision maker. That has the task of assigning received data to each received value pair. Once adjustment has been carried out and if the interference is sufficiently low, the received values match the transmitted data values, apart from a constant delay and an additional rotation of the complex data vector through xc2x190xc2x0 or xc2x1180xc2x0.
Overall, the adaptive CAP receiver includes three control loops, specifically the control of the sampling rate, the control of the digital level adaptation and the control of the filter coefficients. The problem which arises in that case is that stable behavior of the CAP receiver is not ensured in the case of miscoupling of the control loops.
It is accordingly an object of the invention to provide an adaptive cap receiver and a method for controlling a cap receiver, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and which decouple control loops of the CAP receiver in order to ensure stable behavior of the CAP receiver. With the foregoing and other objects in view there is provided, in accordance with the invention, a method for controlling a CAP receiver, which comprises converting an input signal with a clock-controlled A/D converter; providing a digital level-control circuit; providing an adaptive controlled reception filtering system with two parallel filters; providing a decision maker for outputting reconstructed signal coordinates; decoupling the digital level-control circuit and the adaptive reception filtering system by activating either an adjustment of the digital level-control circuit or a coefficient adjustment of the adaptive reception filtering system; and decoupling clock control of the A/D converter and the adaptive reception filtering system.
With the foregoing and other objects in view there is provided, in accordance with the invention, an adaptive CAP receiver, comprising a clock-controlled A/D converter for converting an input signal; a digital level-control circuit; an adaptive controlled reception filtering system with two parallel filters; a decision maker for outputting reconstructed signal coordinates; and control of a sampling clock, a digital level and the adaptive reception filtering system according to the method.
In accordance with another mode of the invention, two ENABLE signals are generated in order to control the decoupling, the first ENABLE signal activating the digital level-control circuit, and the second ENABLE signal activating the coefficient adjustment of the adaptive reception filtering system.
In accordance with a further mode of the invention, the mean value of the two decision errors and the mean value of the reception level are used to generate the ENABLE signals.
In accordance with an added mode of the invention, the mean value for the reception level is compared with a prescribed reference value, with the first ENABLE signal being set to one and the second ENABLE signal being set to zero when the mean value is greater than the reference value.
In accordance with an additional mode of the invention, if the mean value of the received level is smaller than the reference value, the first ENABLE signal is set to zero, although in this case the second ENABLE signal is not set to one until the mean decision error is smaller than a predetermined threshold.
In accordance with yet another mode of the invention, only a four-stage CAP signal is used when operating in a starting phase, that is to say as the CAP receiver is starting up.
In accordance with yet a further mode of the invention, the level-control circuit controls the gain in such a way that a prescribed level value is yielded at the output of the reception filter pair.
In accordance with yet an added mode of the invention, the respective decision error is used to control the coefficient adjustment.
In accordance with a concomitant mode of the invention, the clock control is derived from the values upstream and downstream of the decision maker and from a coefficient of the filter pair.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an adaptive CAP receiver and a method for controlling a cap receiver, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.